find a quantity of merchants sorted by descending item quantity sold

[abreaux26]

• This endpoint should return a variable number of merchants ranked by total number of items sold:
• The URI should follow this pattern: GET /api/v1/merchants/most_items?quantity=x
• where x is the number of merchants to be returned. The quantity should default to 5 if not provided, and return an error if it is not an integer greater than 0.